In a known air-fuel ratio control device, as disclosed in U.S. Pat. No. 4,088,100, for example, an exhaust gas component concentration such as an oxygen concentration in an exhaust gas is detected by an exhaust gas component concentration sensor, and an air quantity or a fuel quantity in a fuel mixture to be supplied to an internal combustion engine mounted in a vehicle is regulated according to a detection value by the exhaust gas component concentration sensor to thereby feedback control an air-fuel ratio of the supplied fuel mixture, for the purposes of purification of the exhaust gas and improvement in fuel consumption.
Generally, just after starting deceleration of the internal combustion engine, an intake manifold vacuum is rapidly increased by the closure of a throttle valve. As a result, fuel deposited on an inner wall surface of the intake manifold is vaporized to render the air-fuel ratio of the fuel mixture to be supplied overrich, causing an increased emission of unburnt harmful components such as HC (hydrocarbon). To prevent such overenrichment of the air-fuel ratio, a known method is provided to supply a so-called shot air (deceleration secondary air) into the intake manifold upon closing of the throttle valve.
However, when rapid opening and closing of the throttle valve is repeated at a low vehicle speed of about 20 km/h, for example, a large amount of the shot air is supplied to the engine, causing an overlean state of the air-fuel ratio. Particularly, at low vehicle speeds, a fluctuation in engine torque with respect to a load fluctuation is rendered large because of a reduced weight of the fuel mixture. As a result, there is a possibility of engine stall due to the overlean air-fuel ratio.